Channel equalization in receivers

ABSTRACT

Systems and techniques relating to channel equalization in received communications signals are described. In one aspect, a communications signal of a channel is obtained and channel effects are removed from the communications signal by generating one or more time-varying channel response estimates.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the priority of U.S. ProvisionalApplication Ser. No. 60/684,093, filed May 23, 2005, and entitled “AnOFDM Receiver for Timing-Varying Channels.”

BACKGROUND

This document relates to channel equalization in receivers.

When a signal is transmitted over a channel, the received signal may bedifferent from the transmitted signal. For example, a signal transmittedwirelessly may be attenuated by the time it reaches a receiver. Thesechanges to the transmitted signal are called channel effects. If, forexample, a transmitter or a receiver are moving during transmission orreception, respectively, the channel effects may include Dopplereffects.

Receivers may employ systems and techniques to model these channeleffects and remove or reduce them through channel equalization.

SUMMARY

According to an aspect of the described systems and techniques, a methodincludes obtaining a communications signal of a channel and removing, inthe frequency domain, channel effects from the communications signal,where the removing includes generating, in the time domain, one or moretime-varying channel response estimates.

In some implementations, the removing channel effects also includesestimating a time-invariant channel response based at least in part onthe communications signal; generating a first estimated signal based atleast in part on the time-invariant channel response; and generating asecond estimated signal based at least in part on a first time-varyingchannel response. Also, the generating one or more time-varying channelresponse estimates can include generating the first time-varying channelresponse based at least in part on the first estimated signal.

In some implementations of the method, the obtaining the communicationssignal includes receiving the communications signal.

In some implementations, the removing channel effects also includesgenerating a second time-varying channel response based at least in parton the second estimated input signal and generating a third estimatedinput signal based at least in part on the second time-varying channelresponse.

In some implementations, the removing channel effects also includesiteratively generating additional time-varying channel responses andadditional estimated input signals until a final estimated input signalis determined to be acceptable based at least in part on one of thegenerated additional estimated input signals.

In some implementations, the estimating a time-invariant channelresponse includes receiving one or more pilot signals on one or moresubcarrier signals.

In some implementations, the generating a first estimated input signalincludes performing channel equalization using the time-invariantchannel response.

In some implementations, the generating the first time-varying channelresponse includes performing a least-mean-squared technique.

In some implementations, the generating a second estimated input signalincludes performing channel equalization using the first time-varyingchannel response.

According to an aspect of the described systems and techniques, a methodincludes: receiving a communications signal comprising one or more pilotsignals on one or more subcarrier signals; estimating a time-invariantchannel response based at least in part on the one or more pilotsignals; generating a first estimated frequency-domain source signalbased at least in part on the time-invariant channel response;generating a first estimated time-domain source signal based at least inpart on the first estimated frequency-domain source signal; estimating atime-varying time-domain channel response using a least-mean-squaredtechnique; generating a time-varying frequency-domain channel responsebased at least in part on the time-varying time-domain channel response;and generating a second estimated frequency-domain source signal basedat least in part on the time-varying frequency domain channel response.

In some implementations, the generating a first estimated time-domainsource signal includes performing a one-dimensional Inverse Fast FourierTransform technique; and the generating a time-varying frequency-domainchannel response includes performing a two-dimensional Fast FourierTransform technique.

According to an aspect of the described systems and techniques, anapparatus includes an input configured to obtain a communications signalof a channel, and a channel equalizer configured to remove, in thefrequency domain, channel effects from the communications signal usingone or more time-varying time-domain channel response estimates.

In some implementations, the channel equalizer includes: a firstequalization unit configured to perform a time-invariant equalizationand to be responsive to a time-invariant channel response; an estimationunit configured to generate a time-varying channel response and to beresponsive to an output of the first equalization unit; and a secondequalization unit configured to perform a time-varying equalization andto be responsive to an output of the estimation unit.

In some implementations, the apparatus also includes a first decisionunit responsive to an output of the second equalization unit.

In some implementations, the channel equalizer also includes a seconddecision unit responsive to an output of the first equalization unit.

In some implementations, the estimation unit includes aleast-mean-squared unit.

In some implementations, the channel equalizer also includes an initialresponse estimator configured to estimate the time-invariant channelresponse based on one or more pilot signals.

In some implementations, the channel equalizer also includes: aone-dimensional Fast Fourier Transform unit; a one-dimensional InverseFast Fourier Transform unit; and a two-dimensional Fast FourierTransform unit.

In some implementations, the input configured to obtain thecommunications signal is configured to receive the communications signalof the channel.

According to an aspect of the described systems and techniques, anapparatus includes a means for obtaining a communications signal of achannel and a means for removing, in the frequency domain, channeleffects from the communications signal, the means for removingcomprising a means for generating, in the time domain, one or moretime-varying channel response estimates.

In some implementations, the means for removing channel effects alsoincludes: a means for estimating a time-invariant channel response basedat least in part on the communications signal; a means for generating afirst estimated signal based at least in part on the time-invariantchannel response; and a means for generating a second estimated signalbased at least in part on a first time-varying channel response. Also,the means for generating one or more time-varying channel responseestimates can include a means for generating the first time-varyingchannel response based at least in part on the first estimated signal.

In some implementations, the means for obtaining the communicationssignal includes a means for receiving the communications signal.

In some implementations, the means for removing channel effects alsoincludes a means for generating a second time-varying channel responsebased at least in part on the second estimated input signal and a meansfor generating a third estimated input signal based at least in part onthe second time-varying channel response.

In some implementations, the means for removing channel effects alsoincludes a means for iteratively generating additional time-varyingchannel responses and additional estimated input signals until a finalestimated input signal is determined to be acceptable based at least inpart on one of the generated additional estimated input signals.

In some implementations, the means for estimating a time-invariantchannel response includes a means for receiving one or more pilotsignals on one or more subcarrier signals.

In some implementations, the means for generating a first estimatedinput signal includes a means for performing channel equalization usingthe time-invariant channel response.

In some implementations, the means for generating the first time-varyingchannel response includes a means for performing a least-mean-squaredtechnique.

In some implementations, the means for generating a second estimatedinput signal includes a means for performing channel equalization usingthe first time-varying channel response.

According to an aspect of the described systems and techniques, anapparatus includes: a means for receiving a communications signalcomprising one or more pilot signals on one or more subcarrier signals;a means for estimating a time-invariant channel response based at leastin part on the one or more pilot signals; a means for generating a firstestimated frequency-domain source signal based at least in part on thetime-invariant channel response; a means for generating a firstestimated time-domain source signal based at least in part on the firstestimated frequency-domain source signal; a means for estimating atime-varying time-domain channel response using a least-mean-squaredtechnique; a means for generating a time-varying frequency-domainchannel response based at least in part on the time-varying time-domainchannel response; and a means for generating a second estimatedfrequency-domain source signal based at least in part on thetime-varying frequency domain channel response.

In some implementations, the means for generating a first estimatedtime-domain source signal includes a means for performing aone-dimensional Inverse Fast Fourier Transform technique and the meansfor generating a time-varying frequency-domain channel response includesa means for performing a two-dimensional Fast Fourier Transformtechnique.

According to an aspect of the described systems and techniques, a systemincludes an antenna configured to obtain a communications signal of achannel and a channel equalizer configured to remove, in the frequencydomain, channel effects from the communications signal using one or moretime-varying time-domain channel response estimates.

In some implementations of the system, the channel equalizer includes: afirst equalization unit configured to perform a time-invariantequalization and to be responsive to a time-invariant channel response;an estimation unit configured to generate a time-varying channelresponse and to be responsive to an output of the first equalizationunit; and a second equalization unit configured to perform atime-varying equalization and to be responsive to an output of theestimation unit.

In some implementations, the system also includes a first decision unitresponsive to an output of the second equalization unit.

In some implementations of the system, the channel equalizer alsoincludes a second decision unit responsive to an output of the firstequalization unit.

In some implementations of the system, the estimation unit includes aleast-mean-squared unit.

In some implementations of the system, the channel equalizer alsoincludes an initial response estimator configured to estimate thetime-invariant channel response based on one or more pilot signals.

In some implementations of the system, the channel equalizer alsoincludes: a one-dimensional Fast Fourier Transform unit; aone-dimensional Inverse Fast Fourier Transform unit; and atwo-dimensional Fast Fourier Transform unit.

In some implementations of the system, the antenna configured to obtainthe communications signal is configured to receive the communicationssignal of the channel.

The described systems and techniques can result in one or more of thefollowing advantages. Channel effects acting on a transmitted signal canbe reduced or removed. Examples of channel effects that may be, reducedor removed include Doppler effects and other time-varying channeleffects.

The described systems and techniques can be implemented in electroniccircuitry, computer hardware, firmware, software, or in combinations ofthem, such as the structural means disclosed in this specification andstructural equivalents thereof. This can include a program operable tocause one or more machines (e.g., a signal processing device) to performoperations described. Thus, program implementations can be realized froma disclosed method, system, or apparatus, and apparatus implementationscan be realized from a disclosed system, program, or method. Similarly,method implementations can be realized from a disclosed system, program,or apparatus, and system implementations can be realized from adisclosed method, program, or apparatus.

For example, the disclosed embodiment(s) below can be implemented invarious systems and apparatus, including, but not limited to, a specialpurpose programmable machine (e.g., a wireless access point, a router, aswitch, a remote environment monitor), a mobile data processing machine(e.g., a wireless client, a cellular telephone, a personal digitalassistant (PDA), a mobile computer, a digital camera), a general purposedata processing machine (e.g., a minicomputer, a server, a mainframe, asupercomputer), or combinations of these.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other features, objectsand advantages may be apparent from the description and drawings, andfrom the claims.

DRAWING DESCRIPTIONS

FIG. 1A is a block diagram of a signal transmission and receptionsystem.

FIG. 1B is a diagram showing channel equalization in a signal (e.g.,orthogonal frequency-division multiplexed (OFDM) signal) transmissionand reception system.

FIG. 2 shows a system for channel equalization.

FIG. 3 shows a process for channel equalization.

FIGS. 4A-4E show various exemplary implementations of the describedsystems and techniques.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

FIGS. 1A and 1B depict a transmission and reception system. A sourcedevice 105 may transmit a signal (e.g., orthogonal frequency-divisionmultiplexed (OFDM) signal) over a channel 125 to a receiver 145.Receiver 145 may obtain the signal via input 143. In someimplementations, if the channel comprises wireless transmission, input143 may be an antenna. In some implementations, e.g., transmission overa cable, input 143 may be an connector attached to the cable. In someimplementations, e.g., transmission via satellite, input 143 may be asatellite reception dish.

A source device 105 may process a frequency-domain source signal X(k)110 to be transmitted. Signal X(k) 110 may be modulated, e.g., byquadrature amplitude modulation techniques such as QAM16 (16-QAM) orQAM256 (256-QAM), or by other modulation techniques. Signal X(k) may betransformed into the time-domain using, for example, an Inverse FastFourier Transform (IFFT) unit 115, resulting in a time-domain sourcesignal x(n) 120.

Time-domain source signal x(n) 120 may be sent over channel 125 and maybe received at receiver 145. Channel 125 may be a conventional radiofrequency channel or it may be another transmission channel, e.g.,cable, satellite, etc. Different channels may modify the signal byintroducing different channel effects. For example, some channels mayattenuate the signal. The received signal y(n) 130 arriving at receiver145 may thus be different from the time-domain source signal x(n) 120.

Components within receiver 145 may introduce noise into the system.Various noise components may be modeled in this description by adding anoise signal n(n) 135 to received signal y(n) 130. Thus, the modeledreceived signal {tilde over (y)}(n) 140 at receiver 145 may berepresented as{tilde over (y)}(n)=y(n)+n(n).  (1)

The modeled received signal {tilde over (y)}(n) 140 may be transformedinto frequency domain signal {tilde over (Y)}(k) 155, for example byusing a Fast Fourier Transform unit 150. The receiver 145 may performchannel equalization 160 to compensate for channel effects, producing anequalized receive signal {tilde over (X)}(k) 165. The equalized receivedsignal {tilde over (X)}(k) 165 may then be processed by a decision block170 (e.g., a Viterbi decoder).

Decision block 170 may be used to remove noise from the equalizedreceived signal {tilde over (X)}(k) 165. For example, suppose atransmitted binary signal consisted of two values, 0 volts and 1 volt.As a result of noise, the equalized received binary signal may comprisevalues other than exactly 0 and 1. Thus, for example, if 0.2 V werereceived, decision block 170 can be used to interpret/decode thereceived signal to be an intended value of 0. Similarly, if 0.8 V werereceived, decision block 170 can interpret/decode the received value tobe the intended value of 1. Decision block 170 may be implemented usingone or more of several algorithms, such as, in one implementation, usinga slicer technique or a Viterbi algorithm. The decoded signal{circumflex over (X)}(k) 175 resulting from decision block 170 may beclose to or identical to frequency-domain source signal X(k) 110.

The channel equalization block 160 may be implemented in several ways.For example, assuming channel 125 has time-invariant channel effects,the impulse response for the channel may be represented as h(l). Thismay be represented in the frequency domain by H(k), which results fromthe N-point FFT of h(l):

$\begin{matrix}{{{{H(k)} = {\sum\limits_{n = 0}^{N - 1}{{h(n)} \cdot w^{k \cdot n}}}},{k = 0},1,{{\ldots\mspace{14mu} N} - 1.}}{w = {{\mathbb{e}}^{{{- j} \cdot 2}{\pi/N}}.}}} & (2)\end{matrix}$Received signal y(n) 130 may then be represented in the frequency domainas:Y(k)=X(k)·H(k).  (3)The frequency-domain representation {tilde over (Y)}(k) 155 of themodeled received signal {tilde over (y)}(n) 140 may then be representedas{tilde over (Y)}(k)=X(k)·H(k)+N(k).  (4)

H(k) can be determined by transmitting a known pilot signal on a set ofsubcarriers. The pilot signal may be in either the time-domain or thefrequency domain. In the frequency-domain, a particular frequency maycontain the pilot signal, which may be specified by a standard, e.g.,the Digital Video Broadcasting (DVB) standard from the EuropeanTelecommunications Standards Institute (ETSI).

Having determined H(k), channel equalization may be performed accordingto the following equation:{tilde over (X)}(k)={tilde over (Y)}(k)/H(k).  (5)

In many applications, however, the channel cannot be considered to betime-invariant. For example, for a moving receiver 145 (e.g., in a car)the Doppler effect may cause channel effects for a wireless channel tobe time-varying. Other factors, such as changes in electromagnetic fieldor changes in temperature may also cause time-varying channel effects.

For time-varying channels, the impulse response of the channel may berepresented as h(n,l), where n represents time and l represents delay.

Because the channel effects are time-varying, the subcarriers are notorthogonal, and the system may be modeled as:

$\begin{matrix}{{{\overset{\sim}{Y}(k)} = {{\sum\limits_{i = 0}^{N - 1}{{X(i)} \cdot {H_{2}\left( {{k - i},i} \right)}}} + {N(k)}}},} & (6)\end{matrix}$where H₂(k,i) is the 2-dimensional FFT of h(n,l) and can be representedas:

$\begin{matrix}{{H_{2}\left( {k,i} \right)}{\sum\limits_{n = 0}^{N - 1}{\sum\limits_{l = 0}^{N - 1}{{h\left( {n,l} \right)} \cdot w^{l \cdot k} \cdot {w^{n \cdot i}.}}}}} & (7)\end{matrix}$

Although H₂(k,i) cannot be accurately estimated by pilot subcarriers, itmay be estimated directly by estimating h(n,l). Equation (6) may bewritten in the time-domain as:

$\begin{matrix}{{\overset{\sim}{y}(n)} = {{\sum\limits_{l = 0}^{L}{{h\left( {n,l} \right)} \cdot {x\left( {n - l} \right)}}} + {{n(n)}.}}} & (8)\end{matrix}$

If x(n) is known, then a least-mean-squared (LMS) algorithm can be usedto adaptively track h(n,l):

$\begin{matrix}{{{{h\left( {{n + 1},l} \right)} = {{h\left( {n,l} \right)} + {\mu \cdot \left( {{\overset{\sim}{y}(n)} - {y(n)}} \right) \cdot {x\left( {n - l} \right)}}}},{l = 0},{{\ldots\mspace{14mu} L} - 1}}\mspace{79mu}{{{where}\mspace{14mu}{y(n)}} = {\sum\limits_{l = 0}^{L}{{h\left( {n,l} \right)} \cdot {x\left( {n - l} \right)}}}}} & (9)\end{matrix}$and where μ is the LMS step size.

FIG. 2 shows a system for estimating the equalized received signal{tilde over (X)}(k) 165. At 205, {tilde over (Y)}(k) 155 is generated bytaking the FFT of modeled received signal {tilde over (y)}(n). At 210,{tilde over (H)}(k) is estimated based on pilot subcarriers in both thecurrent and adjacent symbols. At 215, if the channel is assumed to betime-invariant, {tilde over (X)}(k) may be estimated:{tilde over (X)}(k)={tilde over (Y)}(k)/{tilde over (H)}(k).  (10)In some implementations, a time-varying channel equalization can beapplied at 215 if a time-varying model is available. Suchimplementations may be useful in several technologies, including thoserelated to WiMAX and the Institute of Electrical and ElectronicsEngineers, Inc., (IEEE) Standard 802.16(e).

At 220, a decision block generates {circumflex over (X)}(k), for exampleaccording to a slicer technique or a Viterbi algorithm. At 225, thetime-domain representation {circumflex over (x)}(n) may be generated byimplementing IFFT({circumflex over (X)}(k)).

At 230, h(n,l) may be estimated using {circumflex over (x)}(n) in theLMS algorithm of equation (9). At 235, H₂(k,i) may be generated byimplementing the two-dimensional FFT of h(n,l).

At 240, a time-varying channel equalization is performed using equation(6) based on the estimated time-varying transfer function H₂(k,i)(calculated at 235), signal {circumflex over (X)}(k) (calculated at220), and signal {tilde over (Y)}(k) (calculated at 205). The equalizedsignal may be represented {tilde over ({tilde over (X)}(k).

At 245, signal {circumflex over ({circumflex over (X)}(k) is generatedby implementing a decision procedure on equalized signal {tilde over({tilde over (X)}(k). This decision procedure may be the same as orsimilar to the decision procedure implemented at 220.

An iteration control 250, determines whether to perform anotheriteration using {circumflex over ({circumflex over (X)}(k) (calculatedat 245) as the input to the IFFT at 225. In some implementations, {tildeover ({tilde over (X)}(k) (calculated at 240) may be used instead as theinput to the IFFT at 225. Because iterations may be costly in terms ofboth time and hardware, it may be sufficient to perform only one or twoiterations. In some implementations, an LMS error may be calculatedbetween particular signals of successive iterations (e.g., betweenequalized signals such as {tilde over ({tilde over (X)}(k), decisionsignals such as {circumflex over ({circumflex over (X)}(k), etc.).Iteration control 250 may continue iterations until a particular LMSerror threshold is met. The resulting equalized signal generated at theend of the process may be a statistically close estimate of sourcesignal 110.

Some implementations need not perform all the operations depicted inFIG. 2 or described herein. For example, in some implementations, thedetermination at iteration control 250 of whether to loop back in theprocess is not performed (i.e., single iteration systems).

FIG. 3 shows a process for channel equalization. At 310, acommunications signal of a channel may be obtained (e.g., by receiver145). At 320, a time-invariant channel response may be estimated basedat least in part on the communications signal (e.g., by channelequalization unit 160, unit 210). A first estimated signal may begenerated based at least in part on the time-invariant channel responseat 330 (e.g., by channel equalization unit 160, unit 215, unit 220). At340, a first time-varying channel response may be generated based atleast in part on the first estimated signal (e.g., by channelequalization unit 160, unit 230). At 350, a second estimated signal maybe generated based at least in part on the first time-varying channelresponse (e.g., by channel equalization unit 160, unit 240, unit 245).

At 360, a determination may be made whether to perform another iterationof the process (e.g., by channel equalization unit 160, iterationcontrol 250). If another iteration is to be made, a second time-varyingchannel response may be generated at 340 (e.g., by channel equalizationunit 160, unit 230) based at least in part on the second estimatedsignal, and, at 350, a third estimated signal may be generated (e.g., bychannel equalization unit 160, unit 240, unit 245) based at least inpart on the second time-varying channel response.

Some implementations may not perform some of the operations depicted inFIG. 3 or described herein. For example, in some implementations, thedetermination at 360 of whether to loop back into the process is notperformed (i.e., single iteration systems).

FIGS. 4A-4E show various exemplary implementations of the describedsystems and techniques. Referring now to FIG. 4A, the described systemsand techniques (e.g., associated with receiver 100) can be implementedin a high definition television (HDTV) 420. The described systems andtechniques may be implemented in either or both signal processing and/orcontrol circuits, which are generally identified in FIG. 4A at 422, aWLAN (wireless local area network) interface 429 and/or mass datastorage 427 of the HDTV 420. The HDTV 420 receives HDTV input signals ineither a wired or wireless format and generates HDTV output signals fora display 426. In some implementations, signal processing circuit and/orcontrol circuit 422 and/or other circuits (not shown) of the HDTV 420may process data, perform coding and/or encryption, performcalculations, format data and/or perform any other type of HDTVprocessing that may be required.

The HDTV 420 may communicate with mass data storage 427 that stores datain a nonvolatile manner such as optical and/or magnetic storage devices.The mass data storage 427 may be a hard disk drive (HDD), such as a miniHDD that includes one or more platters having a diameter that is smallerthan approximately 1.8″. The HDTV 420 may be connected to memory 428such as random access memory (RAM), read only memory (ROM), low latencynonvolatile memory such as flash memory and/or other suitable electronicdata storage. The HDTV 420 also may support connections with a WLAN viaa WLAN network interface 429.

Referring now to FIG. 4B, the described systems and techniques may beimplemented in a control system of a vehicle 430, a WLAN interface 448and/or mass data storage 446 of the vehicle control system. In someimplementations, the described systems and techniques are implemented ina powertrain control system 432 that receives inputs from one or moresensors such as temperature sensors, pressure sensors, rotationalsensors, airflow sensors and/or any other suitable sensors and/or thatgenerates one or more output control signals such as engine operatingparameters, transmission operating parameters, and/or other controlsignals.

The described systems and techniques may also be implemented in othercontrol systems 440 of the vehicle 430. The control system 440 maylikewise receive signals from input sensors 442 and/or output controlsignals to one or more output devices 444. In some implementations, thecontrol system 440 may be part of an anti-lock braking system (ABS), anavigation system, a telematics system, a vehicle telematics system, alane departure system, an adaptive cruise control system, a vehicleentertainment system such as a stereo, digital versatile disc (DVD),compact disc and the like. Still other implementations are contemplated.

The powertrain control system 432 may communicate with mass data storage446 that stores data in a nonvolatile manner. The mass data storage 446may include optical and/or magnetic storage devices for example harddisk drives HDD and/or DVDs. The HDD may be a mini HDD that includes oneor more platters having a diameter that is smaller than approximately1.8″. The powertrain control system 432 may be connected to memory 447such as RAM, ROM, low latency nonvolatile memory such as flash memoryand/or other suitable electronic data storage. The powertrain controlsystem 432 also may support connections with a WLAN via a WLAN networkinterface 448. The control system 440 may also include mass datastorage, memory and/or a WLAN interface (all not shown).

Referring now to FIG. 4C, the described systems and techniques can beimplemented in a cellular phone 450 that may include a cellular antenna451. The described systems and techniques may be implemented in eitheror both signal processing and/or control circuits, which are generallyidentified in FIG. 4C at 452, a WLAN interface 468 and/or mass datastorage 464 of the cellular phone 450. In some implementations, thecellular phone 450 includes a microphone 456, an audio output 458 suchas a speaker and/or audio output jack, a display 460 and/or an inputdevice 462 such as a keypad, pointing device, voice actuation and/orother input device. The signal processing and/or control circuits 452and/or other circuits (not shown) in the cellular phone 450 may processdata, perform coding and/or encryption, perform calculations, formatdata and/or perform other cellular phone functions.

The cellular phone 450 may communicate with mass data storage 464 thatstores data in a nonvolatile manner such as optical and/or magneticstorage devices for example hard disk drives HDD and/or DVDs. The HDDmay be a mini HDD that includes one or more platters having a diameterthat is smaller than approximately 1.8″. The cellular phone 450 may beconnected to memory 466 such as RAM, ROM, low latency nonvolatile memorysuch as flash memory and/or other suitable electronic data storage. Thecellular phone 450 also may support connections with a WLAN via a WLANnetwork interface 468.

Referring now to FIG. 4D, the described systems and techniques can beimplemented in a set top box 480. The described systems and techniquesmay be implemented in either or both signal processing and/or controlcircuits, which are generally identified in FIG. 4D at 484, a WLANinterface 496 and/or mass data storage 490 of the set top box 480. Theset top box 480 receives signals from a source 482 such as a broadbandsource and outputs standard and/or high definition audio/video signalssuitable for a display 488 such as a television and/or monitor and/orother video and/or audio output devices. The signal processing and/orcontrol circuits 484 and/or other circuits (not shown) of the set topbox 480 may process data, perform coding and/or encryption, performcalculations, format data and/or perform any other set top box function.

The set top box 480 may communicate with mass data storage 490 thatstores data in a nonvolatile manner. The mass data storage 490 mayinclude optical and/or magnetic storage devices for example hard diskdrives HDD and/or DVDs. The HDD may be a mini HDD that includes one ormore platters having a diameter that is smaller than approximately 1.8″.The set top box 480 may be connected to memory 494 such as RAM, ROM, lowlatency nonvolatile memory such as flash memory and/or other suitableelectronic data storage. The set top box 480 also may supportconnections with a WLAN via a WLAN network interface 496.

Referring now to FIG. 4E, the described systems and techniques can beimplemented in a media player 400. The described systems and techniquesmay be implemented in either or both signal processing and/or controlcircuits, which are generally identified in FIG. 4E at 404, a WLANinterface 416 and/or mass data storage 410 of the media player 400. Insome implementations, the media player 400 includes a display 407 and/ora user input 408 such as a keypad, touchpad and the like. In someimplementations, the media player 400 may employ a graphical userinterface (GUI) that typically employs menus, drop down menus, iconsand/or a point-and-click interface via the display 407 and/or user input408. The media player 400 further includes an audio output 409 such as aspeaker and/or audio output jack. The signal processing and/or controlcircuits 404 and/or other circuits (not shown) of the media player 400may process data, perform coding and/or encryption, performcalculations, format data and/or perform any other media playerfunction.

The media player 400 may communicate with mass data storage 410 thatstores data such as compressed audio and/or video content in anonvolatile manner. In some implementations, the compressed audio filesinclude files that are compliant with MP3 (Moving Picture experts groupaudio layer 4) format or other suitable compressed audio and/or videoformats. The mass data storage may include optical and/or magneticstorage devices for example hard disk drives HDD and/or DVDs. The HDDmay be a mini HDD that includes one or more platters having a diameterthat is smaller than approximately 1.8″. The media player 400 may beconnected to memory 414 such as RAM, ROM, low latency nonvolatile memorysuch as flash memory and/or other suitable electronic data storage. Themedia player 400 also may support connections with a WLAN via a WLANnetwork interface 416. Still other implementations in addition to thosedescribed above are contemplated.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications can be made without departingfrom the spirit and scope of the disclosure. For example, one or more ofthe receivers described above can be embodied in a number of differentways. A receiver may be embodied as part of an integrated circuit on asingle piece of silicon, where one or many circuits can be formed on asingle silicon substrate, and other digital components used for thecommunication can also be formed on the substrate. In addition, however,a receiver can be embodied as discrete components, e.g., defined usinghardware definition language, or by a suitably programmed digital signalprocessor, or in software executed by a general purpose processor. Theprocessor can be configured to simulate the results of the receiver,e.g., as part of a simulation program such as MATLAB™.

In addition, other modifications are possible. For example, it should beunderstood that the described systems and techniques can analogously beused for other kinds of channel equalization. Moreover, while portionsof the implementations have been described as being done in the digitaldomain, it should be understood that these portions could also beimplemented in the analog domain. Also, various operations depicted in aflowchart or other figure can be skipped or performed out of order andstill provide desirable results. Accordingly, all such modifications andother implementations are within the scope of the following claims.

1. A method comprising: obtaining a communications signal of a channel;and removing, by a receiver device, in the frequency domain, channeleffects from the communications signal, the removing comprisinggenerating, in the time domain, one or more time-varying channelresponse estimates based on an initial decision signal represented inthe time domain after being generated, by the receiver device, for thecommunications signal in the frequency domain.
 2. The method of claim 1,wherein the removing channel effects further comprises: estimating atime-invariant channel response based at least in part on thecommunications signal; generating a first estimated signal based atleast in part on the time-invariant channel response; and generating asecond estimated signal based at least in part on a first time-varyingchannel response; and wherein the generating one or more time-varyingchannel response estimates comprises generating the first time-varyingchannel response based at least in part on the first estimated signal.3. The method of claim 2, wherein the obtaining the communicationssignal comprises receiving the communications signal.
 4. The method ofclaim 2, wherein the removing channel effects further comprises:generating a second time-varying channel response based at least in parton the second estimated input signal; and generating a third estimatedinput signal based at least in part on the second time-varying channelresponse.
 5. The method of claim 4, wherein the removing channel effectsfurther comprises iteratively generating additional time-varying channelresponses and additional estimated input signals until a final estimatedinput signal is determined to be acceptable based at least in part onone of the generated additional estimated input signals.
 6. The methodof claim 2, wherein the estimating a time-invariant channel responsecomprises receiving one or more pilot signals on one or more subcarriersignals.
 7. The method of claim 2, wherein the generating a firstestimated input signal comprises performing channel equalization usingthe time-invariant channel response.
 8. The method of claim 2, whereinthe generating the first time-varying channel response comprisesperforming a least-mean-squared technique.
 9. The method of claim 2,wherein the generating a second estimated input signal comprisesperforming channel equalization using the first time-varying channelresponse.
 10. A method comprising: receiving a communications signalcomprising one or more pilot signals on one or more subcarrier signals;estimating a time-invariant channel response based at least in part onthe one or more pilot signals; generating a first estimatedfrequency-domain source signal based at least in part on thetime-invariant channel response; generating a first estimatedtime-domain source signal based at least in part on the first estimatedfrequency-domain source signal; estimating a time-varying time-domainchannel response using a least-mean-squared technique; generating atime-varying frequency-domain channel response based at least in part onthe time-varying time-domain channel response; and generating a secondestimated frequency-domain source signal based at least in part on thetime-varying frequency domain channel response; wherein estimating thetime-invariant channel response, generating the first estimatedfrequency-domain source signal, generating the first estimatedtime-domain source signal, estimating the time-varying time-domainchannel response, generating the time-varying frequency-domain channelresponse, and generating the second estimated frequency-domain sourcesignal are performed by a receiver device.
 11. The method of claim 10wherein: the generating a first estimated time-domain source signalcomprises performing a one-dimensional Inverse Fast Fourier Transformtechnique; and the generating a time-varying frequency-domain channelresponse comprises performing a two-dimensional Fast Fourier Transformtechnique.
 12. A storage device encoded with a program operable to causeone or more machines to perform operations comprising: obtaining acommunications signal of a channel; and removing, in the frequencydomain, channel effects from the communications signal, the removingcomprising generating, in the time domain, one or more time-varyingchannel response estimates based on an initial decision signalrepresented in the time domain after being generated, by a receiver, forthe communications signal in the frequency domain.
 13. The storagedevice of claim 12, wherein the removing channel effects comprises:estimating a time-invariant channel response based at least in part onthe communications signal; generating a first estimated signal based atleast in part on the time-invariant channel response; and generating asecond estimated signal based at least in part on a first time-varyingchannel response; and wherein the generating one or more time-varyingchannel response estimates comprises generating the first time-varyingchannel response based at least in part on the first estimated signal.14. The storage device of claim 13, wherein the obtaining thecommunications signal comprises receiving the communications signal. 15.The storage device of claim 13, wherein the removing channel effectsfurther comprises: generating a second time-varying channel responsebased at least in part on the second estimated input signal; andgenerating a third estimated input signal based at least in part on thesecond time-varying channel response.
 16. The storage device of claim15, wherein the removing channel effects further comprises iterativelygenerating additional time-varying channel responses and additionalestimated input signals until a final estimated input signal isdetermined to be acceptable based at least in part on one of thegenerated additional estimated input signals.
 17. The storage device ofclaim 13, wherein the estimating a time-invariant channel responsecomprises receiving one or more pilot signals on one or more subcarriersignals.
 18. The storage device of claim 13, wherein the generating afirst estimated input signal comprises performing channel equalizationusing the time-invariant channel response.
 19. The storage device ofclaim 13, wherein the generating the first time-varying channel responsecomprises performing a least-mean-squared technique.
 20. The storagedevice of claim 13, wherein the generating a second estimated inputsignal comprises performing channel equalization using the firsttime-varying channel response.
 21. An apparatus comprising: an inputconfigured to obtain a communications signal of a channel; and a channelequalizer configured to remove, in the frequency domain, channel effectsfrom the communications signal using one or more time-varyingtime-domain channel response estimates generated based on an initialdecision signal represented in the time domain after being generated forthe communications signal in the frequency domain.
 22. The apparatus ofclaim 21, wherein the channel equalizer comprises: a first equalizationunit configured to perform a time-invariant equalization and to beresponsive to a time-invariant channel response; an estimation unitconfigured to generate a time-varying channel response and to beresponsive to an output of the first equalization unit; and a secondequalization unit configured to perform a time-varying equalization andto be responsive to an output of the estimation unit.
 23. The apparatusof claim 22, further comprising a first decision unit responsive to anoutput of the first equalization unit.
 24. The apparatus of claim 23,wherein the channel equalizer further comprises a second decision unitresponsive to an output of the second equalization unit.
 25. Theapparatus of claim 22, wherein the estimation unit comprises aleast-mean-squared unit.
 26. The apparatus of claim 22, wherein thechannel equalizer further comprises an initial response estimatorconfigured to estimate the time-invariant channel response based on oneor more pilot signals.
 27. The apparatus of claim 22, wherein thechannel equalizer further comprises: a one-dimensional Fast FourierTransform unit; a one-dimensional Inverse Fast Fourier Transform unit;and a two-dimensional Fast Fourier Transform unit.
 28. The apparatus ofclaim 22 wherein the input configured to obtain the communicationssignal is configured to receive the communications signal of thechannel.
 29. A storage device encoded with a program operable to causeone or more machines to perform operations comprising: receiving acommunications signal comprising one or more pilot signals on one ormore subcarrier signals; estimating a time-invariant channel responsebased at least in part on the one or more pilot signals; generating afirst estimated frequency-domain source signal based at least in part onthe time-invariant channel response; generating a first estimatedtime-domain source signal based at least in part on the first estimatedfrequency-domain source signal; estimating a time-varying time-domainchannel response using a least-mean-squared technique; generating atime-varying frequency-domain channel response based at least in part onthe time-varying time-domain channel response; and generating a secondestimated frequency-domain source signal based at least in part on thetime-varying frequency domain channel response.
 30. The storage deviceof claim 29 wherein: the generating a first estimated time-domain sourcesignal comprises performing a one-dimensional Inverse Fast FourierTransform technique; and the generating a time-varying frequency-domainchannel response comprises performing a two-dimensional Fast FourierTransform technique.
 31. An apparatus comprising: an input configured toreceive a communications signal comprising one or more pilot signals onone or more subcarrier signals; and one or more equalization circuitsconfigured to effect operations comprising: estimating a time-invariantchannel response based at least in part on the one or more pilotsignals; generating a first estimated frequency-domain source signalbased at least in part on the time-invariant channel response;generating a first estimated time-domain source signal based at least inpart on the first estimated frequency-domain source signal; estimating atime-varying time-domain channel response using a least-mean-squaredtechnique; generating a time-varying frequency-domain channel responsebased at least in part on the time-varying time-domain channel response;and generating a second estimated frequency-domain source signal basedat least in part on the time-varying frequency domain channel response.32. The apparatus of claim 31 wherein: the generating a first estimatedtime-domain source signal comprises performing a one-dimensional InverseFast Fourier Transform technique; and the generating a time-varyingfrequency-domain channel response comprises performing a two-dimensionalFast Fourier Transform technique.